The objective of these studies is to utilize viruses isolated from milk and spontaneous mammary tumors of Mus species other than that of M. musculus (the laboratory mouse) to: (1) examine the relationship between spontaneous mammary cancer and type-B retroviruses in these species; and (2) develop immunoassays that detect conserved interspecies determinants of type-B retroviruses to search for related viral gene products in more distant species. Major accomplishments during this past year include: (a) the isolation and characterization of the first type-B retroviruses to be isolated from species other than M. musculus, and (b) the development of the first interspecies RIAs for mammary tumor viruses (MTVs). These assays, for the major envelope proteins and the major internal protein of MTVs have determined the distribution of these and related gene products in other species. These assays employ both conventionally raised heterologous sera and monoclonal antibodies generated to conserved antigenic determinants with interspecies reactivities. These studies have expanded the spectrum within which MTV etiologically related gene products can be analyzed.